Migrating from GtkOptionMenu and GtkCombo to GtkComboBox and
GtkComboBoxEntry

Prior to 2.4, GTK+ offered two widgets for the task of selecting one
item from a list of options. GtkOptionMenu presents the list of
options as a menu while GtkCombo presents them in a Windows-style list
popup. The only difference between the two is that a GtkCombo allows to
manually edit the selected value, while the GtkOptionMenu does not.

In GTK+ 2.4, a unified API for list selection was introduced, with
GtkComboBox for the non-editable case and GtkComboBoxEntry for the
editable case.
The selection of the display style — menu or list —
is no longer done at the API level, but has been made themeable via
the style property
GtkComboBox::appearance.